1. Field of the Invention
This invention relates to an electromagnetically driven shutter and more particularly to an electromagnetically driven shutter arranged to have a shutter driving rotor rotate on the optical axis of a photograph-taking lens.
2. Description of the Prior Art
There have been known electromagnetically driven shutters arranged to have a shutter driving rotor rotated on the optical axis of a photograph-taking lens with a conductor coil pattern provided on the shutter driving rotor. Such electromagnetically driven shutters include a type having a magnetic flux applied in the direction of the optical axis of a photograph-taking lens. To efficiently produce a driving torque for rotating the rotor, the shutter of this type has been arranged to have an electric current flow in a radial direction from the rotation center of the rotor.
The conventional shutter driving rotor having the conductor coil provided thereon as stated above is arranged as shown in FIGS. 1 and 2 of the accompanying drawings. Referring to these drawings, a rotor 1 is formed to have circumferential parts 1a, radially extending parts 1b and outer peripheral parts 1c. On both sides of the rotor 1, there is provided a conductor coil 2 which is arranged along the periphery of the rotor 1. There are provided shutter driving pins 3a, 3b and 3c which are secured to the rotor 1. In FIG. 2, a reference symbol L indicates the length of the effective portions of magnetic fluxes obtained from permanent magnets, which are not shown. The direction in which an electromagnetic force F is exerted is indicated with an arrow.
As apparent also from these illustrations, the form of the conventional conductor coil 2 causes the electric current to flow in the radial direction from the rotation center of the rotor 1. Therefore, the driving force required for rotating the rotor 1 can be effectively produced. On the other hand, in accordance with this conventional arrangement, the peripheral parts 1a of the rotor 1 in which the electromagnetic force F is not generated becomes larger. This has been causing an increase in the moment of inertia of the rotor 1 and thus resulted in insufficient angular acceleration and poor dynamic characteristics. Besides, the conventional arrangement also has caused the conductor coil 2 to be of a large size, which in turn has caused increase in electric resistance to present a problem also in the electrical aspect.